How important is a Canteen?

I bought a military canteen with a carrying pouch and cup for about $8 (is that a good price?) The canteen and cup are a bit used, but the pouch is new. I wanted to buy a canteen to carry with me when I go camping, but I also have a larger 3 gallon jug on a strap, that I use to carry water. Is it important to have a smaller canteen to carry with you, when you venture away from your camp?